Plural modulation system



L. ESPENSCHIED.

PLURAL MODULATION SYSTEM.

APPLICATION FILED MAR 18, 1920.

Patented Dec. 7, 1920.

2 SHEETS-SHEET I.

I c 6'] 4/ I 275-525 INVENTOR.

Lia 70mm I L. ESPENSCHIED.

PLURAL MODULATION SYSTEM.

APPLICATION FILED MAR. 18, I920.

Patented Dec. 7, 1920.

2 SHEETSSHEET 2.

IN V EN TOR.

E I I l BY llZy vwm ATTORNEY UNITED STATES PATENT OFFICE LLOYD ESPENSCHIED, OF QUEENS, NEW YORK, ASSIGNOR TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK.

21.11am. monnna'rron SYSTEM.

Specification of Letters Patent.

Patented Dec. '7, 1920.

Application filed March 18, 1920. Serial No. 366,802.

To all whom it may concern:

Be it known that I, LLOYD Esrnnsomno, residing at Queens, in the county of Queens and State of New York, have invented certain Improvements in Plural Modulation Systems, of which the following is a specification.

This invention relates to'the transmission of telegraph signals and more particularly to transmlssion of telegraph signals by $0 called carrier currents.

One of the objects of this invention is to narrow the band of frequencies employed in transmitting telegraph signals by carrier waves in a system in which modulation takes place by a plurality of steps.

Another object of the invention is to suppress the transmission of carrier waves durmg certain signaling conditions, by taking advantage of the property of a vacuum tube whereby when operated upon any non-amplifying portion of its characteristic it will function to practically prevent the transmission of carrier currents supplied thereto.

Another object of the invention is to pro-' vide for the transmission of signals formed by direct current pulses bysuppressing the transmission of carrier waves during the steady state condition of the pulses and only transmitting carrier waves change from one steady state to another.

A further object of the invention is to suppress, in a system for translating so-called direct current signalsxinto carrier waves, the direct current component of the signal and transmit only the alternating current component.

Other and further objects of the invention will be clear, from the following description when read in connection with the appended drawing, in which Figures 1 and 2 represent circuit diagrams of two forms of transmitting arrangements which may be employed in connection with the invention, Fig. 3 of which is a diagram of a receiving arrangement to be used in connection with the transmitting circuit of F i 2, and Figs.

illustrating the operation of the modulator em loyed.

eferring to Fig. 1, L designates a transmitting circuit over which tele raph or other direct current signals may e trans- I mitted. The circuit L terminates in a transduring the mitting relay, TR, which controls the application of potential from a battery C to the grid of a modulator M, of the well known vacuum tube type. A idbattery C is connected between the ament and grid of the modulator M,, through a resistance 10, thereby giving the grid a normal potential, so as to cause the tube to operate upon a desired part of its characteristic curve, in a manner more fully described hereinafter.

In order to narrow the band of frequenc1 es involved in the transmission of ordinary direct current signals such, for instance, as telegraph'signals, a filter LF is included in the input circuit of the modulator, as indicated. This filter is preferably a low pass filter of the type disclosed in the United States Patents to George A. Campbell, Nos. 1,227,113 and 1,227,114, issued May 22, 1917.

Anordinary dlrect current signal having a sharp -wave form and recurring at a frequency of say 50 cycles per second actually re resents a frequency band of considerable width and the impressed wave may be analyzed in accordance withFouriers theorem into a series of harmonics of the fundamental frequency of say 50 cycles. In order to transmit the signal it is not necessary to transmit all of these harmonics, the extent to which they may be eliminated being de- "pendent, among other things, upon the type of receiver employed, a wave-recording type of receiver, such as the photographic or siphon recorder, for instance, requn'mg'a less sharp wave and, therefore, the transmission of fewer harmonics than a relay t pe of receiver. Consequently, by properly esigning the filter LF as to its upper cutoff fre uen'cy, all of the harmonics above a g'ven reque'ncy may be eliminated, thereb narrowing the band of frequencies which it is necessary to transmit in order to reproduce the signal. It will always be necessary to transmit at least the fundamental or dot frequency, so that the limit to which this expedient may be extended will be to eliminate all of the harmonics and retain simply the fundamental. I

he modulator M may be supplied .with carrier waves from a suitable source G which is illustrated as being a vacuum tube oscillator; The fre uency of this source should not be too big as compared with the frequency to be transmittedthrough the filter LF, for the reason that the side bands resulting from modulation should be sepa-' rated by a suflicient amount relative to the carrier frequency to permit of the selection of one side band to the exclusion of the other.

In order to select one of the side bands, say the upper side band appearing in the output ('lIOUlt of the modulator M,, a filtering arrangement F is included'between the modulator M and the next translating element of the circuit. This filter may be, for instance a fairly sharply tuned circuit capable of transmitting frequencies between 200 and 250 cycles, where the highest frequency employed in the signaling operation is 50 cycles. A modulator, M of vacuum tube or other well known type, is included in the circuit beyond the filter F and is supplied with carrier waves from a source G This modulator serves to modulate the carrier waves from the source G in accordance with the band of frequencles selected by the filter F thereby stepping up the selected band in frequency. A filter F which may be similar in construction to the filter F is included in the circuit be ond the modulator M to select the upper side band in preference to the lower side band. By designing this filter to transmit a band between 2200 and 2250 cycles, for instance, both the carrier frequency of 2,000 c cles and the lower side band extending rom 1750 to 1800 cycles will be spaced from the upper side band a sufficient distance to be substantially suppressed.

In order to still further step up the frequency, an additional modulator, M may be provided, said modulator bein supplied with carrier waves of a still higher frequency from the source G The frequency of this source is illustrated as being 20,000

cycles. The out ut circuitof the modulator M is coupled t rough a transformer 11 to a' transmitting antenna A, and in order to select the upper side band appearing in the output circuit of the modulator M to'the exc usion of the carrier frequency and the lower side band, the antenna And the output circuit of the modulator M may be, tuned so as to readily transmit frequencies in the neighborhood of 22,200 to 22,250 cycles.

The modulators M and M operate in the usual and well understood manner. The modulator M,. however, is adjusted to transmit the carrier frequency applied thereto only under certain conditions. The operation of this modulator may be understood by referring to the curves in Figs. 4 to 7 inclusive. In Fig. 4 curve 20 indicates the characteristic curve of a three-element vacuum tube, that is a curve representing the variation of plate current w th applied grid potential. In this curve positive grid voltages are plotted as abscissae to the right of the vertical axis 0 and negative rid voltages are'plotted as abscissa: to the le 't of the axis 0, while the corresponding current values are plotted as ordinates. It will be seen that ast e negative grid voltage is increased the plate current decreases until it becomes zero 1n the neighborhood of the vertical line N.

As the grul voltages increase in a positive direction to the ri ht of the axis 0 it will also be seen that t e current increases until a voltage in the nei hborhoodof the vertical line P IS I'GRClIGd. eyond this point further increase in voltage results in no further increase in the current and the current remains constant at this value, which is known as a saturation value. The points X and Y upon the curve 20 are known as the knees of the curve, the former bein usually called the lower knee and the latter being the upper knee. The portion of the curve between the points X and Y is its amplifying portion and variations in grid potential over this part of the curve result in correspondin variations in the plate current, so that i the tube is working over this part of the curve carrier waves will be transmitted. The two fiat parts of the curve beyond the points X and Y, however, represent ortions of the characteristic curve in w ich the tube is practically unresponsive to potential variations, since the output current is constant at either zero or its saturation value. If the tube is operating upon either one of these two portions of its characteristic it will not transmit carrier waves.

If now the battery C of the modulator M in Fig. 1 be given such a value as to bring the rid potential to the point designated by the dotted line M, the normal adjustment of the tube will be about the midpoint of its amplifying characteristic. If, with the grid potential thus adjusted, an alternating potential of carrier frequency be impressed upon the grid, a corresponding variation in the plate current will occur, so that carrier waves will be transmitted.v Suppose now the battery C of Fig. 1, which is controlled by the transmitting relay TR is connected to the grid circuit and is so adjusted that its potential when, superposed upon that of the battery C will bring the grid potential to the point N, if an alternating potential be superposed upon this grid potential it will be observed that no variation in current flow -will take place in the plate circuit, since over this portion of the characteristic no current flow takes place. When the potential of the batterv C is removed by the action of the relay TR,-the rid potential will be shifted to t e point where, as already ointed 0111i), transmission of carrier waves Wlll take ace.

The variation in grid potential with time. corresponding to the opening and closing of dotted line curve ampli I waves are transmitted. The variation m .representin time and the circuit basis.

the circuit of'the battery C at the relay TB in response to signals is-indicated by the 1 of 6 and the superpiosed alternating potential fromthe source is indicated at the curve 22 of. Fig. 6. These curves are plotted on the same scale as Fig. 4, grid potentials being lotted as abscissaa and time being plotte as ordinates. The correspondin late current is indicated in Fig-6, whic is likewise plotted onthe same scale as Fig. 4, ordinates gate current being plotte as absclssae. y causing this current to flow through the primary of a carrier frequency transformer the alternating carrier current component will...appear in the secondary circuit, this alternating component bein indicated by the curve in Fig. 6". It will e observed from the curves of Fig. 6 and Fi 6 that no current is transmitted when t e tube is working on the flat part of the curve, but that carrier waves are transmitted when its operation is shifted to the am lifying portion of its characteristic as in ioated m Fig. 6. ubstantiallg' similar results ma be secured by so a justing the battery to o erate upon the portion of the curve to t e right of the point Y. Upon this part ofthe curve the p ate current remains steady at its saturation value, but when the otential of the battery C is removed by t e trans-. mittin relay the operation is shifted to the f ying portion of the curve and carrier potential, .due to opening and closing the circuit of the battery C, will now be as indicated by the dotted line curve 23 of Fig. 7 and the su erposed alternating current potential is in ioated by the curve 24. The corresponding plate current is indicated in Fig. 7 where it will be noted that the plate current is steady while the grid potential varies about an axis in the neighborhood of the vertical line P of. Fig. 4, but is decreased to a pulsating value at 25 when the battery C is disconnected and the tube operates on the amplifying portion of its characteristic. The alternatin carrier current component is indicated in ig. 7".

The transmitting relay TR may be operated on either an 0 en circuit or a closed As above described, carrier waves are tran smitted when the relay TR 0 erates to open the circuit of the battery U and the carrier waves are suppressed when the circuit of the battergeis closed. Exactly theopposite effect may produced by suitabl proportioningl the batteries G and C. I for examp1le,t e battery C is so adjusted as to. bring t e normal gridpotential to the point N in Fig. 4 and the battery C is oppositely poled and so adjusted that when it 1s connected in circu t the grid potential is brought to the pomt M, then it the batte' so adj uste that when included in circuit the will beobserved that carrier transmission takes places as in Figs. 6 6 and 6", when the circuit of the batte 0 is closed. Likew1se,""1f the battery is adjusted to bring the normal grid potential to the point P and C is then oppositely poled and grid potential is brou ht tothe point M, carr1er transmission wil only take place when the battery "(3' is in circuit, as. indicated in the curves 7 7', and 7".

In general, the operation of the circuit of Fig. 1 is as follows: Telegraph signals from the circuit L o erate the transmitting re- -may involve merely the fundamenta frequency. These pulses produce otential changes in the grid circuit, whici results in shlfting its operation to different points of the characteristic, as indicated by the curves 21, and 23 of Figs. 6 and 7, respectively, so that carrier waves are alternately transmitted and suppressed, depending upon whether the pulse shifts the operation of the tube to the amplifying or non-amplifying portion of its characteris tic. In the output circuit of the modulator M, upper and lower side bands appear, the upper band ranging from sa 200 to 250 cyc es and the lower side ba from 150 to 200 cycles. The upper side band may be selected b the filter F and transmitted to the modu ator M,, where it modulates os-' cillations of say 2,000 cycles from the source G Upper and lower side bands again apear in the output: circuit of the modulator the upper ing selected by the filter F and transmitted to the modulator M H ere again modulation of a frequency of say 20,000 cycles from the source Gr occurs and the u per side band is selected by the antenna for transmission. Instead of the antenna A an ordinary wire transmission line ma be used as indicated in dottedlines. T e filters F, and F in selecting in each instance the upper side band, also operate to suppress the carrier frequencies as well'as the lower side bands.

Fig. 2 illustrates the invention as applied to a multiplex circuit. The a paratus for the second and third steps 0 modulation in this figure may be the same as in Fig. 1, although the filter F is illustrated as being a band filter .of the Campbell type above so proportioned and adjusted as :to transmit a proper band of frequencies. The transmitting relay TR, filter LF,, modulator M and source G, associated with the first channel are identical with corresponding apparatus of Fig. 1 and the frequency assigned to this channel'maybe the same as that assi ned to the channel of Fig. 1, so that the lter F should be designed to pass a band lying between 200 and 250 cycles. The relatively low carrier frequency assigned to the second channel should be somewhat different from that assigned to the first channel a'i'ld is illustrated as being 275 cycles, the frequency of the generator G,,. Assuming that the signaling frequency is the same as that of Fig. 1, say 50 cycles per second, the filter F should e designed to transmit a band from 275 to 325 cycles. In a similar manner the generator (i of the third channel may have a frequency of 350 cycles and the filter F, pass a band from 350 to 400 cycles.

The second channel of Fig. 2 diifers somewhat from the first channel and is so arranged that carrier waves are only transmitted during the change from one steady state to the other. For this purpose the transmittingrelay TR is arranged to connect either one or the other of two batteries of opposite polarity to the grid circuit of the modulator M The operation of the modulator M under these circumstances may be understood by reference to the diagrams in Figs. 4, 5, 5' and 5 As described in connection with Fig. 1, the battery 0,, of the modulator M is so poled and adjusted as to bring the norma grid potential to the intermediate point M. One of the batteries under control of the relay TR when connected in the grid circuit, then shifts the resultant grid potential to the point P, while the other battery is so poled and adjusted as to shift the grid potential to the point N.

Let us suppose now that the transmitting relay TR, has its armature so thrown as to bring about the last mentioned result. The carrier waves from the source G will not be transmitted, as the plate current will remain at zero. This will be clear from Figs. 5 and 5. In Fig. 5 the dotted line curve 26 indicates the slow speed changes in grid potential, due to the action of the transmitting relay TR, while 27 is a plot of the superposed high frequency potential from the source G,,. Turning now to the curve of Fig. 5, it will be seen that in the part 28 of the curve the current remains zero, regardless of the high fIEK uency variation in potential indicated in Fig. 5. As soon, however, as the relay TR operates to connect in the other battery, so that the grid potential will be as indicated at P in Fig. 4, the tube will momentarily operate over amplifying portion of its characteristic, so

that high frequency pulsations a ain occur, as indicated at 31 of Fig. 5. pon reaching the initial steady state condition,.however, the current remains zero and no high frequency is transmitted. The alternating ,carrier current, component of the ulsating current indicated in the curve of ig. 5" is shown in Fig. 5 and, as indicated, consists of a short train of high frequency im ulses corresponding to each change in o larity produced by the transmittmg re ay. In

each instance, as soon as the change-over is com leted and the steady state condition reac 18d, regardless of whether the potential of the grid be as indicated at N or P, the alternatlng current will'cease.

By the arrangement just described the carrier waye is only transmittin during the change-over and is not transmitted during -the steady state of the low frequency signal. The same result may be secured by meansof the arrangement shown in connection with the third transmittin channel, in which the modulator M is o the du lex vacuum tube type illustrated and described in the U. S. application of John R. Carson, Serial No. 157,413, filed March 15, 1917. Such a duplex vacuum tube arrangement normally suppresses the carrier by reason of the fact that the carrier source'Gr is connected to the common branch of the two similar input circuits. The signaling battery C,, is connected in a local circuit including the filter LF,,, which local circuit is inductively connected to the modulator M,,' through a transformer 33. The transformer 33 suppresses the direct current component of the low frequency si nal, so that the steady state condition r0 uces no effect upon the modulator M pon either closing or opening the circuit of the battery 0' however, one alteration is induced through the transformer 33 and unbalances the modulator circuit,.so that a brief train of high frequency waves from the source Gr is transmitted.

The receiving apparatus for use in connection with the transmitting systems of Figs. 1 and 2 may be as illustrated in Fig. 3 in which AR desi nates a receiving antenna, D a demodu ator or detector supplied with modulating oscillations from a source RGr and D, designates a second demodulator or detector supplied with modulating currents from a source RG RF,

menace designates truer which may be, m stancel, of the Campbell tyge, said filter beemodulators D,

an D,. This filter is designed to suppress] the up r side band resulting from the first;

r transmit in inserted between the step 0 demodulation and eel the lower side band. The wi th of the lower side band will of course, be, sufficient band has been passed through this filter, the

several bands making up the composite band ma be so arated by means of filters RF,, R and F in the individual receivmg channels. For instance; the filter RF may select a band lying between 200 and 250 cycles, this band being impressed upon a receiving detectorD, su plied with detecting current from a source and having an ordinary receiving relay'li-R in its output circuit. The filter RF, may select a band between 275' and 325 cycles, this band,being impressed upon a detector D, supplied with a detecting frequency from a sourceG In a similar manner the filter RF, may select a frequency between 350 and 400 cycles, this band being impressed u 'on a demodulator D,", which is supplied 51th a detecting frequency from a source The operation ofthe receiving circuit of Fig. 3 when used in connection with the transmitting circuit of Fi 1 may be as follows: Let it be assume that the maximum low frequency signal transmitted by the relays TR, TR etc., in Fig. 2, will not exceed 50 cycles per second, in the first channel the modulator M will produce two side bands on either side of- 200 cycles, of which the upper side band from 200 to 250 c cles ma be selected by the filter F,. In tlie secon channel, the modulator M will produce two side bands on either side of the carrier frequency of 275 cycles and the upper side band from 275 to 325 cy les may be selected by the filter F,,. In the third channel the modulator M,., will produce side bands on either side of the carrier frequency of 350 cy les and the upper side band from 350 to 400 cycles may be selected by the filter F These several bands now coalesce and form a single band extending from 200 to 400 cycles, this band being impressed upon the modulator M,. As a result, the new carrier frequency of 2,000 cycles, as well as two side bands, the lower of which extends from 1600-cycles to 1800 ycles and the upper of which extends from 2200 band is transmitted by the filter The lower side 7 cycles to 2400 cycles, apfiear in the output circuit of the modulator -balgdtanfd the carrier frequency of 2,000

or side The -transmitted side band is then impressed upon the'modulator M,, so that up er and lower sidebands are again produce The lower side ,band extends from 17,600 to 17,800

zcycare suppressed and the u cycles, while the upper side band extends from 22,200'to 22,400 cycles. By tuning the antenna to the average frequency of the up- ..per band, say 22,300 cycles, the upper band will be readily radiated, while the carrier frequency of 20,000 cycles of the lower side band may be substantially eliminated.

The radiated band extending from 22,200

to 22,400 cycles is now received by the re 'ceiving antenna ,AR, which is tuned to the intermediate frequency and the received band is modulated by a frequency of 20,000 cycles sugplied to the detector D from the source R The lower side band resulting from this operation extending from 2200 cycles to 2400 cycles may be selected by means of the filter RF and im ressed upon the detector D where a modu ating action with the frequency of 2000 cycles takes lace. This results in a further step down in frequency,-the lower side band extending from 200'to 400 cycles being selected by the filter RF,. The several bands making up this composite low frequency lected by means of filters F RF, and RF,,", each being designed to pass the band indicated, and the selected bands are. again other, it is necessary to provide a special form of signal receiving relay for the corresponding receiving channels in order that the receiving apparatus. will discriminate between a change from positive to negative and a change from negative to positive, both of these changes being represented by the transmission of the brief train of high frequencylwaves. Such a receiving relay is indicated at RR and BB in Fig. 3. Bela RR is a polarized relay having two win ings,'34 and 35. Normally the armature 36 of the relay will rest on one of its conta ts, so that current may flow through one of the windings, say 35, from the battery 37. The battery 37 is of low potential and merely supplies enough current to hold the armature against the pole of the relay'to which it is shifted. Suppose now a train of waves representing a change-over from positive to negative is being received, the detector D wi l translate this brief tram of waves into i a .pulse of current flowing in its output circuit. This pulse will pass through the transformer 38, producing a pulse of current through the winding 35 1n a direction opposite to that produced from the battery 37. This pulse of current is of considerable strength and therefore tends to throw the armature 36 to the opposite contact of the relay. The pulse being of momentary duration has now ceased and a current from the battery 37 flows through the other winding- 34, in such a direction as to hold the armature against the opposite contact. If now a second train of waves corresponding to a chan e-over from negative to positive is receive d, an exactly similar )ulse of current will appear in the output circuit of the detector 1),. Owin to the shifted connections of the relay R, however, this )ulse now flows through the winding 34 o the relay and being of greater strength and opposite in direction from the current flowing from the battery 37, shifts the armature of the relay back to its original position. Thus it will be seen that since changes from negative to positive always succeed chan es from positive to negative, in regular or er, the relay RR will in effect discriminate between the two kinds of changes, since, in response to a pulse of current, it will first produce one action and then in response to a similar pulse of the same character will produce another action.

The receiving a paratus of the first channel of Fig. 3 may be used in connection with the transmitting apparatus of Fig. l or in connection with the transmitting apparatus of the first channel ofFig. 2. The receiving a paratus associated with the other two ciiannels of Fig. 3 can only be used in connection with the second and third transmitting channels of Fig. 2. The several frequencies referred to in the precedin 'description are to be understood as eing merely illustrative, a wide variation in the frequencies used being possible in actual practice. I

It will further be obvious that the general principles herein disclosed may be embodied in many other organizations widely different I from those illustrated, without departing from the spirit of the invention as defined in the following claims.

What is claimed is:

1. The method of translating low frequency telegraph currents into high frequency carrier currents for carrier current transmission, which consists in curbing the low frequency telegraph impulses to eliminate therefrom the higher harmonics, thereby narrowing the band of low frequencies representing the telegraph signal, and then stepping up the frequency of the resultant currents by successive'steps of modulation with carrier currents.

2. The method of translating low frequency currents into high frequency carrier currents for carrier current transmission, which consists in passing thelow frequency telegraph currents through a filter to eliminate the higher harmonics, then raising the frequency of the resultant currents by successive steps of modulation.

3. In a signaling system, means for reducing tele raph impulses, a filter for e iminating the igher harmonics from said telegraph impulses, and a plurality of modulatmg devices, oneworki'ng into the other and each supplied with carrier current whereby, upon the application of the currents transmitted through the filter to the first modulating device of the series, such low frequency currents will be raised to a desired carrier frequenc by a plurality of steps.

4. The metho of transmitting telegraphic impulses by means of carrier waves, which consists in supplying carrier waves to a vacuum tube so adjusted that under a certain condition of operation the tube will work upon a non-amplifying part of its characteristic curve which lies outside of one of the two knees'thereof, thereby substantially preventing the transmission of carrier waves, and causing by means of telegraph impulses such a change in the adjustment of the tube as to cause it to operate upon the amplifying part of its characteristic, thereby transmitting carrier waves.

5. The method of transmitting telegraphic impulses by means of carrier waves, which consists in impressing carrier waves upon a vacuum tube so adjusted that normally it operates upon a non-amplifying, flat portion of its characteristic curve lying beyond one of the knees of said curve, so that normally carrier currents will not be transmitted. and in disturbing the normal adjustment of the tube by means of telegraphic impulses so that the tube operates upon the amplifying portion of its characteristic curve to transmit carrier currents.

6. The method of modulating carrier currents in accordance with telegraphic impulses through the agency of a vacuum tube comprising a heated filament, a grid and plate electrodes, which consists in maintaining under certain conditions such an adjustment of the grid potential that the tube operates upon a non-amplifying substantially fiat portion of its characteristic curve, im ressing carrier currents upon the tube, an varying the grid potential in accordance with telegraphic impulses to cause the tube to operate at certain times upon the amplifying portion of its characteristic wire, whereby the transmission of carrier waves will be permitted when the wire is operated,

" um tube mo istic beyondon'e of the knees thereof, where- I by the transmission of carrier waves is substantially prevented, and means to disturb the adjustment of said tube by the action of telegra hic impulses, so that the tube works upon t e amplifying portion of its characteristic, thereby transmitting carrier waves.

8. In a telegraph signaling system, a vacuulator having a filament grid and plateelectrode, means to supply carrier waves to said tube, the grid potential of the said tube, whereby during either steady state tube being normally so adjusted that it operates u on a non-amplifying substantial y fiat portion of its characteristic curve, thereby substantially preventing the transmission of carrier waves, means responsive 'to telegra h impulses to vary the potential of the gri so that the tube operates upon the amplifying portion of its characteristic, therey transmitting carrier waves. f

9. The method of translating direct current ulses .into carrier waves, which consists 1n transmitting carrier waves during the period that the direct current pulse is undergoing a change, while suppressing the transmission of carrier waves during the con-- transmitted during achange in the applied potential, and in applying direct current pulses to'said device to produce changes in the applied potential, thereby permitting the transmission of carrier waves during such changes only.

11. The methodof translating direct current pulses into carrier waves, which consists 1n impressing the direct current ulses upon'a vacuum tube modulator so ad usted that during one steady state of the pulses the tube will operate upon a non-amplif ing portion of its characteristic lying beyon one nee of the curve, while during another steady state condition of the ulses, the tube will operate upon a non-amp ifyinghportion of its characteristic lying beyond e other knee of the curve, an

waves upon said tube whereby durin either steady state condition of the applie pulses.v

substantially no transmission of carrier waves will take place, while during the transition from one steady state to the other, the

modulator having grid filament and plate in impressing carrier tube will pass through the am lifying portion of its characteristic with consequent transmission of carrier waves.

12. The method of translating direct current pulses into carrier waves, which consists in applying potentials corresponding to said pulses to the grid circuit of a vacuum tube modulator comprising filament grid and plate electrodes, said tube bein so adjusted that the potential on its gri during one steady state condition of the applied pulses will be such that the tube operates on anonamplifying part of its characteristic beyond one knee of the curve, while during the other steady state condition of the pulses, the potential on the grid will be such that the tube operates on a non-amplifying. portion of its characteristic lying be 0nd the other knee of the curve, and in app ying carrier waves to condition of the pulses, substantially no transmission of carrier waves takes place, but during the transition from one steady state condition to the other a change in the potential of the grid takes place which causes the tube to pass through the amplifying portion of its curves, thereby transmitting carrier waves during such transition.

13. In a signaling s stem,'-'a vacuum tube 95. electrodes, a source of carrier waves therefor, means to app] potentials to said grid corresponding to cirect current pulses the tube being so adjusted and the potential applied during one steady state condition of the direct current pulses being such that the tube operates on a non-amplifying portion of its characteristic beyond one of the knees of the curve, and the potential corresponding to the other steady state condition of the 106 pulses bein such that the tube operates on a non-ampli ying portion of its characteristic beyond the other knee of the curve, whereby carrier waves are substantially suppressed during the stead state periods, and are 110 transmitted only uring the transition from one steady state to the other.

14. In a multiplex telegraph system, a plurality of carrier channe s, means to a ply telegraphic impulses to said channels, fi ter- 116 ing means in each channel for eliminating the higher harmonics ofthe impulses, thereby narrowing theband of frequencies representing the tele raph signals, means in each channel to mod ulate re atively low carrier 120 frequencies in accordance with the telegraphic pulses passed through said filtering means, andmeans to raise the resultant carvrier waves to a higher frequency by a plurality of subsequent modulating steps.

In testimony whereof I have signed my name to this specification this 16th day of March 1920. y

' 'LLOYYD ESPENSOHIED. 

